1. Field of the Invention
This invention relates generally to an apparatus for producing a video signal and, more particularly, is directed to a video projecting apparatus for projecting a compound color image onto a projection screen.
2. Description of the Prior Art
Color television systems of the projection type are well-known in the art. Typically, in such systems, three primary color images, that is, red, green and blue images, are separately projected by three cathode ray tubes and combined on a projection screen to produce a compound color image. More particularly, in such systems, the red, green and blue color components of a received video signal are each applied to a respective cathode ray tube for modulating the respective electron beam as the beam scans the color phosphor screen of that tube. The resultant single color images formed on the color phosphor screens are then superimposed on a projection screen by a plurality of lenses to form a compound color image. However, such a system has proven to be undesirable for various reasons. For example, the utilization of three separate cathode ray tubes results in a video projection system of a relatively large size. Due to this relatively large size, and due to the use of three cathode ray tubes, the system has proven to be very costly. In addition, the power requirements in this system are large because of the utilization of three electron guns.
Other types of television systems have attempted to remedy the disadvantages of the above video projection system. For example, commonly employed television receivers, not of the projection type, typically include a single cathode ray tube with three electron guns. Such receivers, in order to obtain accurate color registration of the produced image, utilize repetitive red, green and blue color phosphor stripes or triads of phosphor dots along the front face of the cathode ray tube. Consequently, to obtain synchronization of the electron beams from the three guns with the respective color phosphor stripes or dots, a complex synchronization system must be used. Further, in order to block stray electrons from impinging on the wrong color phosphor dots or stripes, the tube must utilize a shadow mask or aperture grill, resulting in additional cost and complexity. Again, as with the three tube-three gun projection systems, the power requirements in this system are large.
Although single cathode ray tube-single electron beam systems have been devised, such systems have also proven to be undesirable for various reasons. For example, one such system is a beam-index color television receiver in which complex synchronization circuitry must be utilized to synchronize the color signals which modulate the single electron beam as it scans the repetitive color phosphor stripes. To accomplish this, repetitive index stripes are provided on the face of the cathode ray tube in predetermined positional relationships to the color phosphor stripes and emit light or radiant energy as they are scanned by the electron beam. A photo-detector is operative to detect light emitted from the index stripes and to produce a periodic index signal. This index signal is then used to produce a gating signal to sequentially gate the respective color components of the video signal to the cathode ray tube to modulate the electron beam as it scans the respective color phosphor stripes.
Although other single cathode ray tube-single electron beam systems have been proposed, these systems have also been of a relatively complex nature. Thus, for example, in U.S. Pat. No. 2,518,199, a single cathode ray tube-electron beam system is provided in which the electron beam scans vertically arranged sequential color posphor screens in a zig-zag manner. However, because of the non-conventional scan utilized in this system, a non-standard NTSC signal must be provided at the video signal transmitter in order for the television receiver to operate properly. Another single cathode ray tube-single electron beam apparatus is shown in U.S. Pat. No. 3,473,872 in which the cathode ray tube has green, red and blue color filters sequentially arranged on the front face of the cathode ray tube in the horizontal line scanning direction. In this system, however, the horizontal scanning frequency is reduced to one-third of its usual value so that only one color component is separated from the video signal during each horizontal line scan. That is, each horizontal line scan produces only one color image. Although such a system is adequate for slow moving or still-motion pictures, the system does not satisfactorily operate with fast-motion pictures because of the flicker effect. This is due to the fact that a conmplete picture is produced only during each one-tenth of a second, rather than during the conventional one-thirtieth of a second. In addition, this apparatus provides a complex fiber-optic network for reproducing the compound color image from the three color phosphor screens of the cathode ray tube.